In light water reactor (LWR) designs, fuel is typically formed into oxide pellets, which consist of uranium oxide or mixed uranium/plutonium oxide. These pellets are then placed in long tubes called cladding tubes to form fuel rods. The cladding tubes form a barrier against radioactive fission products released in the fuel pellets during irradiation.
Proper fuel design requires an economical fuel cycle, while providing the fuel characteristics necessary for safe plant operation. Thus, structural materials must be selected that have low neutron cross-section and low cost, while providing adequate mechanical corrosion resistance characteristics. Fuel assembly design should accordingly allow for the operation of the reactor at the design power and for the highest possible burn-up without breaching the cladding and releasing radioactive products to the primary coolant.
Zirconium alloys are used in fuel designs because they combine desirable nuclear, physical and mechanical properties. Because nuclear-grade zirconium is expensive, its alloys are used only in the active zone of the nuclear core where its neutron economy is most advantageous. Zircaloy-2 and Zircaloy-4 are two slightly different alloys which were developed for nuclear applications. Zircaloy-2 typically contains about 1.4 wt. % tin, 0.15 wt. % iron, 0.1 wt. % chromium and 0.06 wt. % nickel, 1,000 ppm oxygen and the balance zirconium. Zircaloy-4 typically contains about 1.4 wt. % tin, 0.21 wt. % iron, 0.11 wt. % chromium, 30 ppm nickel, 1,200 ppm oxygen and the balance zirconium. Pressurized water reactor (PWR) fuel rods are typically made with Zircaloy-4 cladding, while boiling water reactor (BWR) fuel rods typically utilize Zircaloy-2.
Continuous operation of a reactor requires that the core remain critical. However, to compensate for the gradual depletion of fissile material with time, as burn-up accumulates, and to compensate for other phenomena such as the buildup of fission products, excess reactivity must be built into the nuclear core. This excess reactivity must be controlled at any given time to keep the reactor critical for steady-state operation. This task is accomplished by the use of materials that are strong neutron absorbers or "poisons." Control elements constructed from neutron absorbers regulate power generation according to demand, provide quick shutdown, account for short-term and long-term reactivity changes that result from temperature changes, and adjust for fission product accumulation and fissile material depletion.
The foremost characteristic of a control material is its neutron absorption properties. These vary with the energy of the impinging neutrons but one can gather together the detailed absorption features into a "thermal absorption cross-section," which is of interest in LWR's. The dominant absorber used in control rods in LWR's is boron.
In addition to the movable control rods used in all LWR's, present LWR designs utilize burnable poisons. These are solid neutron absorbers which are placed in the reactor. As the burnable absorber material is subjected to neutron irradiation, it is gradually depleted. Thus the depletion of the burnable poison corresponds, roughly, to the depletion of fissile material. Burnable-poisons are used to counterbalance excess reactivity at the beginning of the fuel cycle and to provide a means for power shaping and optimum core burn-up. Burnable poison compounds currently of interest include boron, gadolinium and erbium.
For additional general background materials on fuel rods, claddings and absorber materials, see, e.g., Frank J. Rahn et al., A Guide to Nuclear Power Technology, (1984).
Many LWR fuel designs employ burnable absorbers in a number of ways. For example, in some designs, burnable absorber rods are placed in fuel assembly lattice locations, thereby displacing fuel rods. Other designs employ burnable absorber rod inserts and fuel assembly guide thimbles. Still other designs involve the formation of burnable-absorber coatings on the inside diameters of cladding tubes, on fuel pellet surfaces, or involve distribution of the burnable absorber within the fuel pellet.
The use of a burnable-poison which is disposed on the inside surface of the fuel cladding tube has several advantages. For example, such a configuration can be used with uranium dioxide fuel pellets provided inside the cladding so that the fuel rod produces as much (or almost as much) power as a regular fuel rod.
Moreover, the burnable-poison can be applied to the cladding tube prior to the introduction of uranium dioxide pellets into the tube, allowing the burnable-poison to be applied to the cladding in a cold (non-nuclear) area. This allows the burnable-poison to be applied by the tubing fabricator or by the fuel-rod fabricator and should reduce the costs associated with the manufacture of the cladding tubes containing the burnable poison.
Furthermore, when the burnable poison is applied to the inside of the fuel cladding tubes, it is relatively easy to adjust the axial gradient of the burnable poison. This provides an advantage over associated methods, which involve putting burnable poison on the pellet and mixing pellet types, by reducing the inventory costs of carrying differing pellet types.
Finally, the use of cladding tubes having a burnable-poison layer provides for improved quality control. For example, the burnable-poison coating depth can be accurately determined by bombarding the tubing with neutrons and measuring the fraction of the neutrons which are not absorbed by the burnable poison.